Although there is consensus that amblyopia can be treated effectively in young children, many eye care practitioners believe that treatment beyond a certain age is ineffective. Some clinicians have believed that a treatment response is unlikely after the age of 6 or 7 years, while others have considered age 9 or 10 years to be the upper age limit for successful treatment. The American Academy of Ophthalmology Preferred Practice Pattern for amblyopia recommends treatment up to age 10 years. The opinion that amblyopia treatment is ineffective in older children may have arisen because the age of 6 to 7 years is thought to be the end of the "critical period" for visual development in humans. This belief, however, was not based on adequate prospectively-collected data.